1. Field of the Invention
The present invention relates to a piezoelectric transformer-inverter which generates an AC voltage by using a piezoelectric transformer and its control circuit as well as its driving method, and particularly to a piezoelectric transformer-inverter optimal to adjust the brightness of a backlighting cold cathode fluorescent lamp for a color liquid crystal display and its control circuit and its driving method.
2. Description of the Prior Art
A piezoelectric transformer is a voltage converting element which generates mechanical oscillation by means of an AC voltage applied to an electrode on the primary side by using the piezoelectric effect of a piezoelectric element and takes out the resulting voltage from an electrode on the secondary side. Compared with an electromagnetic transformer, a piezoelectric transformer has such advantage that it is compact in size and thin in thickness. It is a noteworthy element which can be used as an inverter to ignite a cold cathode fluorescent lamp and as a high voltage power supply.
FIG. 1A shows a schematic perspective diagram of a piezoelectric transformer of third-order Rosen type. A piezoelectric transformer 1 comprises a strip-shaped piezoelectric ceramic plate 1d, a primary electrode 1a, and a secondary electrode 1b. The primary electrode 1a is polarized in a vertical direction. The secondary electrode 1b is longitudinally polarized. In FIG. 1A, an arrow P shows the polarization direction. When an AC voltage V.sub.d is applied to the primary electrode 1a, it oscillates in a vertical direction. Such oscillation generates a longitudinal oscillation of compressional wave. As a result, the piezoelectric ceramic plate 1d oscillates at a resonance frequency depending on a sound velocity and a shape. As a result, the voltage on the secondary electrode 1b can be stepped up in proportion to the thickness of the piezoelectric ceramic plate 1d and a distance between both electrodes. FIG. 1B shows an equivalent circuit of the piezoelectric transformer 1. As shown in FIG. 1B, a driving voltage V.sub.d supplied through a driving circuit 5 is stepped up by means of the piezoelectric transformer 1 and sent to a load 2 as a voltage V.sub.O.
A ratio to the output voltage V.sub.O and the driving voltage V.sub.d corresponds to a step-up ratio of the piezoelectric transformer 1. Meanwhile, the piezoelectric transformer 1 is characterized that the step-up ratio is raised at a range of the resonance frequency as shown in FIG. 11 because the piezoelectric transformer 1 is operated by using resonance. It is also known that the step-up ratio and the resonance frequency of the piezolelectric transformer 1 change on the basis of impedance of the load 2.
In the case of a piezoelectric transformer-inverter using such piezoelectric transformer, it is desirable to achieve a wide brightness adjustment range in the brightness adjustment of a cold cathode fluorescent lamp while suppressing the occurrence of an audible sound. For example, in the case of a backlight for a 9.4 inches color liquid crystal display panel, it is necessary to output an AC wave of about 500V.sub.rms in a tube voltage and about 5 mA.sub.rms in a tube current for a cold cathode fluorescent lamp of 220 mm in length and 3 mm.phi. in diameter as a load. On the other hand, the higher the resonance frequency is, the smaller the physical shape of the piezoelectric transformer becomes. So it is preferable to set a driving frequency at about 100 kHz. If a conductive reflector, etc. is arranged around the cold cathode fluorescent lamp, a stray capacity is formed between the AC current flowing through such lamp and the reflector. Therefore, a great current flows and a brightness is high on a high voltage side of the cold cathode fluorescent lamp, but the current is less and the brightness is uneven on a low voltage side because of a current flowing through the stray capacity in the lamp.
With the increase of a driving frequency and with the decrease of the current flowing through the lamp, this problem becomes worse and worse. To solve this problem, a PWM (Pulse Width Modulation) system is used to adjust the brightness. This is a method to vary the brightness of a cold cathode fluorescent lamp equivalently by turning on and off such lamp in such a quick period that the eyes could not sense flickering.
Japanese Patent Application Laid-open No.8-107678 (1996) filed by the assignee of the present invention discloses one of brightness adjustment techniques by this method. FIG. 2 shows a block diagram of a circuit configuration for driving a piezoelectric transformer disclosed in such patent application. As shown in this FIG. 2, in this circuit, the primary electrode 1a of the piezoelectric transformer 1 is driven with the AC voltage V.sub.d by means of the driving circuit 5, and a stepped-up AC voltage V.sub.O is applied from the secondary electrode 1b of the piezoelectric transformer to the load 2 such as a cold cathode fluorescent lamp. The output current i.sub.o flowing through the load 2 is sent to a load current comparing circuit 3 to be compared with a reference voltage V.sub.ref. If such output current i.sub.o is lower than the reference value V.sub.ref, the load current comparing circuit 3 sends out to a frequency sweep oscillator 4 a signal that directs the frequency sweep oscillator 4 so as to decrease a driving frequency for the driving circuit 5. In this case, the driving frequency is basically set in a frequency range higher than a frequency for making the step-up ratio on a maximum rate according to frequency characteristic shown in FIG. 11. Therefore, to decrease the driving frequency generates a decrease of the step-up ratio whereby the output current i.sub.o is increased. If such output current i.sub.o is greater than the reference value V.sub.ref, the load current comparing circuit 3 sends out to the frequency sweep oscillator 4 a signal that directs the frequency sweep oscillator 4 so as to increase the driving frequency for the driving circuit 5. That is, the frequency sweep oscillator 4 sends out a driving signal which increases and decreases the value of a desired output current i.sub.o with respect to a generated driving frequency for the driving circuit 5. Since the driving circuit 5 drives the piezoelectric transformer 1 in accordance with such signal, the output current i.sub.o from the load 2 is kept at a predetermined value.
The frequency sweep oscillator 4 can set the uppermost and lowermost limits of the sweep frequency of a signal to be sent to the driving circuit 5. By so doing, the driving frequency of the piezoelectric transformer 1 which is outputted from the driving circuit 5 is limited between its uppermost and lowermost values. When the output current i.sub.o does not reach the predetermined value for a certain period, the driving frequency is reduced to the lowermost limit. Therefore, the driving frequency is swept up to the uppermost limit in a short period by a high-speed frequency sweep and then swept down to the lowermost limit again gradually. As described above, by controlling the driving frequency of the piezoelectric transformer 1, and by changing its voltage step-up ratio, the predetermined output voltage V.sub.O can be supplied to the load 2.
The brightness adjusting circuit 6 generates a frequency of several hundred Hz which is considerably lower than the driving frequency of the piezoelectric transformer 1 and does not give a flickering effect to the eyes. This circuit outputs a binary brightness adjusting signal the duty ratio of which changes in proportion to an input brightness adjusting voltage. When the brightness adjusting signal level becomes high, the driving circuit 5 stops the driving operation of the piezoelectric transformer 1 and stops the supply of the load current. The frequency sweep oscillator 4 operates so as to maintain the driving frequency constant while the output current does not flow. When the brightness adjusting signal level becomes low and the driving circuit 5 starts the driving operation of the piezoelectric transformer 1, the frequency sweep oscillator 4 operates so as to maintain the output current i.sub.o constant.
FIG. 3 shows an operation waveform of the above-mentioned brightness adjusting method. As shown in FIG. 3, the brightness adjusting signal is changed over to high and low levels at a period of several hundred Hz. When the brightness adjusting signal level is high, the piezoelectric transformer 1 does not output the driving voltage V.sub.d, so both output voltage V.sub.O and the output current i.sub.o are not supplied. To maximize the brightness of the cold cathode fluorescent lamp (in this figure, the load 2), it is necessary to set the brightness adjusting signal level low always. Conversely, to lower such brightness, it is necessary to keep the brightness adjusting signal level high for a longer period. As described above, the brightness of the cold cathode fluorescent lamp can be adjusted by changing the period of the current supply to the load, namely, the cold cathode fluorescent lamp, by varying the duty ratio of the output signal from the brightness adjusting circuit 6.
Japanese Patent Application Laid-open No.9-107684 (1997) field by the assignee of the present invention discloses a combination of the above-mentioned brightness adjusting circuit and the driving circuit shown in FIG. 2 incorporating a voltage control circuit which absorbs a fluctuation of an input voltage V.sub.DD so as to keep the driving voltage V.sub.d of the piezoelectric transformer 1 constant in spite of the change of a DC input voltage V.sub.DD. FIG. 4 is a block diagram of a circuit configuration for driving a piezoelectric transformer disclosed in such patent application. However, in FIG. 4, some sections, names, symbols, etc., of the block diagram disclosed in such patent application are changed for the purpose of comparing such circuit configuration with that of one embodiment of the present invention described later.
The driving circuit for the piezoelectric transformer 1 shown in FIG. 4 comprises four large circuit blocks, namely, a load current comparing circuit 3 and a frequency sweep oscillator 4 which detect the current i.sub.o flowing through the load 2 or the voltage V.sub.O across the load 2 connected with the secondary electrode 1b of the piezoelectric transformer 1, compare such current or voltage with a predetermined value V.sub.ref, and control the driving frequency of the piezoelectric transformer 1 to maintain the current i.sub.o and the voltage V.sub.O constant; a voltage step-up circuit 8 which generates an AC voltage of the driving frequency from the DC input voltage V.sub.DD in accordance with a driving signal from the above-mentioned current comparator 3 and frequency sweep oscillator 4 and applies such AC voltage to the primary electrode 1a of the piezoelectric transformer 1; a driving voltage controlling circuit 70 which keeps the driving voltage in the form of a sine wave to be applied to the piezoelectric transformer at a predetermined value in spite of a fluctuation of the DC input voltage V.sub.DD ; and a brightness adjusting circuit 6 which controls the current flowing through the load 2 with the PWM. This circuit configuration basically corresponds to a combination of the conventional circuit configuration according to the prior art shown in FIG. 2 and the driving voltage control circuit 70. The driving circuit 5 shown in FIG. 2 comprises the step-up circuit 8 and the driving voltage control circuit 70. This driving voltage control circuit 70 absorbs the change of the input voltage V.sub.DD and keeps the driving voltage of the piezoelectric transformer 1 (the voltage to be applied to the primary electrode 1a) constant.
The configuration and the operation of each circuit block shown in FIG. 4 will be described in detail later in the description of the embodiments of the present invention, so such description is omitted here.
Japanese Patent Application Laid-open No.61-177163 (1986) discloses a power supply equipped with a soft start circuit as the know-how relating to one of the objects of the present invention, namely the suppression of the audible sound when turning on the power. However, such invention does not directly relate to an inverter using a piezoelectric transformer. The power supply disclosed in such patent application is so configured that a ringing choke converter of self oscillation type can perform soft start to suppress the audible sound when turning on the power. This circuit configuration will be described below with reference to FIG. 5A.
The control circuit 109 limits the base current of a transistor 108 and subtracts a value proportional to the received brightness level of a phototransistor 118 from a value proportional to the winding voltage of a feedback winding 123. The phototransistor 118 and a light emitting diode 117 form a photocoupler. When the power is turned on and the output voltage V.sub.O is increasing, a capacitor 115 is charged with the current flowing through the light emitting diode 117, resistor 104 and diode 113 and with the current from a resistor 102. When the light amount of the light emitting diode 117 is large, the control circuit 109 decreases the width of the turning on operation of the transistor 108. When the capacitor 115 is charged, an operational amplifier 110 keeps the output voltage V.sub.O constant by means of a voltage stabilizing circuit which operates according to a reference voltage 116. Thus, as shown in FIG. 5B, by the addition of the soft start circuit (comprising the resistors 102, 103, 104, diodes 112 and 113, and capacitor 115), the load becomes light and the audible sound duration is shortened. As shown in FIG. 5C, the output voltage V.sub.O increases gradually.
The brightness adjusting art disclosed in the above-mentioned Japanese Patent Application Laid-open No.8-107678 (1996) has disadvantages given below. The first disadvantage is that even though the piezoelectric transformer 1 is driven at the frequency of about 100 KHz above the audible frequency range, if such transformer starts and stops the operation repeatedly in order to adjust brightness at a period of several hundred Hz, it produces an audible sound. This is because the driving voltage applied to the piezoelectric transformer 1 becomes a tone burst wave like the driving voltage V.sub.d shown in FIG. 3 if brightness is adjusted with the PWM system. Such waveform is a sine wave but contains broad frequency component in addition to the driving frequency component at the start and stop of the driving operation because the sine wave is periodically modulated with 0 voltage. The piezoelectric transformer 1 produces a parasitic vibration with many nodes as well as an vibration with three nodes N shown in FIG. 1A. The piezoelectric transformer 1 vibrates at the three nodes N because the piezoelectric transformer 1 resonances according to the driving frequency which produces a resonance in third order longitudinal mode. Therefore, there may be other resonance frequencies by which the piezoelectric transformer 1 is resonated, for example in quintic or septimal mode. Moreover, transversal vibration may be also generated. When the piezoelectric transformer 1 is driven by the tone burst wave which contains a driving frequency component other than the driving frequency in third-order longitudinal mode, this driving frequency component produces a parastic vibration of the piezoelectric transformer 1. In the case of this piezoelectric transformer of third-order Rosen type, it is possible to support it with less mechanical loss by physically supporting three nodes N. However, when the above-mentioned brightness adjusting art is employed, the vibration is momentarily disturbed at the start and stop of the driving operation. In this case, such vibration is transmitted to the support mechanism 25 of the piezoelectric transformer 1. As a result, the piezoelectric transformer 1 and its support mechanism 25 vibrate and produce the audible sound.
The second disadvantage is that it is difficult to design the support mechanism 25 for supporting the nodes N so that the piezoelectric transformer 1 can vibrate freely. Because such transformer 1 and support mechanism 25 are mounted on a circuit base plate of an inverter, so it is difficult to predict the physical period of its entire vibration prior to the design. It is also very difficult to prevent a resonance frequency of the support mechanism 25 including the circuit base plate from resonating with the parasitic vibration of the piezoelectric transformer 1.
The third disadvantage is that it is impossible to apply the audible sound prevention know-how by means of the soft start disclosed in Japanese Patent Application Laid-open No.61-177163 (1986) to the piezoelectric transformer. In the case of the know-how described in the above-mentioned patent application, the output voltage for a converter which outputs a DC voltage increases gradually to charge a smoothening capacitor. As a result, the load becomes light when turning on the power and the decrease of the self oscillation frequency of the ringing choke system into the audible range can be prevented. However, if an inverter outputs an AC voltage and controls the driving frequency so as to drive it in a frequency range above an audible range, and if a cold cathode ray tube whose load impedance changes suddenly at the start and stop of discharge is connected as a load, it is difficult to increase the output voltage gradually. In addition, moreover, as a load impedance becomes high just before the cold cathode fluourescent lamp starts discharge, a step-ratio of the piezoelectric transformer 1 becomes high as shown in FIG. 11 whereby a output voltage from the piezoelelectric transformer 1 becomes high. Therefore, if a piezoelectric transformer whose step-up ratio changes greatly due to the load impedance is used, its control becomes more difficult.
The invention disclosed in the above-mentioned Japan Patent Application Laid-open No.9-107684 (1997) has the same disadvantages.